Dry granular solids are notorious for size-segregation whenever they are poured from one container to another. Any time dry granular solids undergo shear flow in a gravity field (or when a body force has a component perpendicular to the direction of shear), large particles tend to rise to the top and fines tend to sift to the bottom. On a particle-scale, two major mechanisms contribute to this size segregation. One is a dynamic-sieve phenomenon whereby the more-mobile fine particles can move into openings created below them faster that large particles can, and thus tend to ‘migrate’ toward the bottom of the vibrated bed. As a consequence of this, under certain vertical vibration conditions, large particles rise to the top of a vibrated packed granular bed (i.e., the so-called ‘Brazil nut’ problem described by Rosato et al, 1987). The second particle-scale mechanism, contributing to size-segregation in shearing flows, comes from larger particles rotating and ‘rolling’ up-and-over smaller particles in the shearing flow. As Haff and Werner (1986) describe, in simulations of shear motion of a granular bed, the effective grain-grain friction coefficient μ is a critical parameter in this kind of sorting process, since if μ is too small, a large spherical grain cannot get sufficient purchase to roll without slipping. Irregularly shaped particles invariably rotate whenever the granular bed shears (the average rotation rate of particles in a shearing flow is proportional to the local shear-rate in the bed). The dynamic-sieve mechanism acts during shearing flows, but it also operates even in non-shearing situations. Vibratory-screen sifters make use of the natural tendencies for granular solids to have the fines migrate to the bottom in vibratory and/or shearing flow. When a container is vibrated or oscillated, the fine-fraction will migrate to the bottom, and by placing a fine screen on the bottom surface, gravity and contact forces from the granular bed above push the fines through the screen, leaving the coarse fraction on top of the screen. Under reduced gravity conditions, such as during planned in situ resource recovery operations on the moon, however, vibratory-screen sifters do not function well for separation of small (i.e., sub 0.15 mm range) particles, as was recently demonstrated in reduced-gravity flights (Ramé et al, 2010).
Trommel Screens (e.g., slowly rotated, nearly horizontal, cylindrical screens, partially filled with granular material) have been used for centuries for screening or size-separation of granular solids. They function very well terrestrially for segregating out large (>1 cm) particulates. They are used extensively in the mining and mineral industry to remove oversize rocks from feed stock before processing steps. Most Trommel screens are designed for dry granular materials; however, there are liquid-solid-slurry versions of Trommel screens that are used to separate out denser ores from lighter material. For dry materials, as the particle size gets smaller or cohesive materials are involved, Trommel screens become less effective, and other configurations are often utilized. The rotation rate of a Trammel screen cannot be increased beyond the ‘critical’ value where centrifuging of the solids onto the outer wall begins, because centrifuging stops the shearing flow in the usual Trommel Screen configuration.
For separations involving smaller particle sizes (down to 0.1 mm or so) one variety of small commercial size-separation units, which are known as ‘centrifugal-sifters’ or ‘centrifugal-screens,’ utilize relatively rapidly rotating paddle-like blades inside of stationary cylindrical screens to separate fines from coarse dry solids. Companies such as Russel Finex Ltd., UK; Kason Corporation, Millburn, N.J.; S. Howes, Inc., NY; or Jiangyin Wanlu Machine Mfg., Jaingsu, China, all sell ‘centrifugal’-sifters or screens which work on nearly the same principle. These ‘paddle wheel’ sifters have a rapidly rotating, slightly-spiral paddle-wheel, or set of blades which shear material over a stationary screen. The rotation rate of the paddle blades is high enough that centrifugal force moves the fines to the stationary outer cylindrical screen. The oversize material is retained inside the mesh screen and moved towards the oversize outlet (at the opposite end of the cylindrical screen from the inlet feed) because of the slight spiral angle of the paddle blades, and is ejected from the machine. These paddle-wheel centrifugal sifters are effective; however, part of their effectiveness may come from the effects of entrained air circulated by the paddle blades. Also, they may have inherent wear problems when utilized with highly abrasive materials. Since some of their effectiveness may come from air entrainment, the effectiveness of such sifters under vacuum conditions, as might be encountered in space-mission applications, has not been established.
Most rotating-screen centrifugal systems involving solids are two-phase fluid-solid systems. The most common centrifugal separators for solids do not have rotating mechanical parts, but instead have stationary mechanical hardware with flowing two-phase mixtures, such as gas-solid cyclone separators or hydro-cyclones, used primarily to separate solids from gases or liquids, respectively, but can also function to select specific size particulates.
Separating liquid from solids, often referred to as de-watering, is often accomplished through the use of rapidly rotating cylindrical or conical outer screens or porous walls designed to allow the liquid to pass through, while retaining the solids inside. The most familiar example might be the spin cycle of a typical clothes washing machine, where the wash water passes through the holes in the rotating cylindrical outer wall of the wash tub and the clothes are retained inside. Rotating mechanical screen-scroll centrifuges are routinely used to dewater coal and other wet particulates. These centrifuges operate at very high rotation rates and typically have conical (10° to 30° half-angle cones) or cylindrical outer screens with screen openings ranging in size from 0.25 mm to 1 mm (these are used extensively in the coal and/or paper industries). Their primary application is liquid-solid separation—allowing liquid to pass through the outer screen wall while retaining the desired particulate product inside the screen.
These dewatering systems are often configured in continuous-flow mode with mixed liquid-solid material entering the system and separate solid and liquid streams leaving the equipment (with the solids often conveyed by a continuous helical screw). In a batch-mode operation, large industrial centrifuges are also commonly used in water and wastewater treatment to dry sludges (creating a dry cake-bed, which is periodically removed).
Dewatering centrifuges utilize both cylindrical and conical screens, and can be oriented in horizontal or vertical configurations, depending on the design and application. In continuous mode operation the cylindrical screen centrifuges utilize either rotating helical (screw) blades to move the dried solid cake material to the exit region, or a traveling scraper blade. Conical screen centrifuges can use a straight ‘peeler’ blade, a tapered helical scroll blade, or they can employ vibrations to convey the drying cake material from the small radius to large radius portion of the conical screen (and the exit). Pre-acceleration of the slurry, up to centrifugal wall speeds, before entering the screen section has been shown to increase screen life significantly (Leung, 1998).
Vibrating conveyance in conical dewatering centrifuges operates by having a high frequency excitation force which produces either axial or circumferential vibration, which in turn, partially ‘fluidizes’ the material on the wall, lowering its effective wall friction and allowing it to move axially toward the larger radius region (Leung, 1998). The half-angle of the conical screen in dewatering centrifuges is usually between 18° and 25°; however, screen suppliers typically stock conical screens for centrifuges with half-angles ranging from 10° to 30°. Recent reviews of various centrifugal options for dewatering solids, describe systems similar to those in Leung's 1998 handbook (Sutherland, 2005; Records and Sutherland, 2002) and point out that most continuous centrifuging processes involving solids utilize centrifuging conical screens, wherein the solids move from the small to the large end of the screen, either by vibration or with mechanical assistance (i.e., utilizing stationary or moving blades, or tapered screws). There appear to be no commercial dry solids separators based on rotating screen centrifuges; although the screen bowl centrifuges used to dry coal slurries have been studied as potential size-separator devices (Mohanty, et al, 2008); however, the discussions of such designs in the literature were still describing systems which utilize particles in a slurry, with water.